Most airborne radars are pulsed radars. Pulsed radars can detect and acquire targets such as airborne, ship borne or ground targets by transmitting radio frequency pulses (also referred to as signals), receiving radio frequency pulses that are reflected from the targets and analyzing the received radio frequency pulses.
The amplitude of the received radio frequency pulses is proportional to R−4 (R being the radial distance between the radar antenna and the target) as well as to the radar cross section of the target. The frequency of the received radio frequency pulse is affected by a frequency shift that is proportional to the frequency of the transmitted radio frequency pulse and to the relative velocity between the radar and the target (what is known as the Doppler shift). The delay between a transmission of a radio frequency pulse and the reception of that radio frequency pulse is proportional to 2*R.
In a typical scenario a target is illuminated by one or more sequences of radio frequency pulses and reflects multiple sequences of radio frequency pulses. The radar analyzes the received radio frequency pulses by correlating between (i) the delay period between a transmission and a reception of the reflected radio frequency pulses. If the change of distance does match the velocity, the radar characterizes the detected target as a normal target. (ii) the velocity (and even acceleration) of the target as reflected by the Doppler shift frequency of the received sequences of radio frequency pulses, and (iii) the change of distance between the radar and the target as reflected by the change in amplitude of the received sequences of radio frequency pulses. In order to simulate a target, various conditions should be fulfilled. Some of these conditions include correlating between the frequency shift of a received radio frequency pulse, the delay period between a transmission and reception of the radio frequency pulse and the amplitude difference between the transmitted radio frequency pulse and the received radio frequency pulse.
U.S. Pat. No. 6,067,041 of Kaiser, et al. titled “Moving target simulator” describes a ground-based system for testing a radar system. The system is positioned in the field of view of the radar and is capable of simulating a target that can radially move along a fixed angle in relation to the radar. The system is relatively large and may include a personal computer.
The system of Kaiser includes: (i) a radio frequency receiver, (ii) a digital radio frequency memory in electrical communication with the radio frequency receiver for storing a signal received thereby, (iii) a digital delay circuit in electrical communication with the digital radio frequency memory for providing a time delay between reception and transmission of the radio frequency signal, (iv) an amplitude modulation circuit in electrical communication with the digital radio frequency memory for modulating an amplitude of the radio frequency signal, (v) a Doppler modulation circuit in electrical communication with the digital radio frequency memory for providing a Doppler-modulated signal that represents a simulated target with simulated linearly changing velocity wherein the simulated range changes parabolically, and (vi) a radio frequency transmitter for transmitting the radio frequency signal after it has been delayed, amplitude-modulated, and/or Doppler-modulated.
The following patents and patent applications, all being incorporated herein by reference describe prior art systems that are used to train radar users: U.S. Pat. No. 5,223,840 of Cronyn titled “Low cost radar target simulator for remote radar testing”; U.S. Pat. No. 4,686,534 of Eddy titled “Retro directive radar and target simulator beacon apparatus and method”; U.S. Pat. No. 5,457,463 of Vencel et al. titled “Radar return signal simulator”; U.S. Pat. No. 4,450,447 of Zebker et al. titled “Synthetic aperture radar target simulator”; U.S. Pat. No. 4,424,038 of Tingleff et al. titled “Inflight aircraft training system”, U.S. Pat. No. 5,428,530 of Brown et al. titled “Airborne reactive threat simulator”; U.S. Pat. No. 5,431,568 of Fey et al. titled “Radar target generator”; U.S. Pat. No. 3,254,340 of Sealander titled “Apparatus for coupling a return signal simulator to a radar unit”; U.S. Pat. No. 4,644,357 of Schaaf et al. titled “Radar clutter simulator”; U.K. patent application GB2405761 of Mussell et al. titled “Complex radar target simulator”; Russian patent RU2193747 of Chubar et al titled “Air target simulator” and Japanese patent JP1253787 of Matsuura et al titled “Reproducing method for dummy visual field for training simulator”.
A radar antenna generates an antenna beam that scans in azimuth and in elevation to cover a desired field of view. Typical azimuth scan patterns cover a range of multiple degrees (for example, twenty, sixty, eighty degrees) while typical elevation scan patterns include one or more bars, equivalent to multiple degrees (for example, three four and ten degrees)
The scanning is typically achieved by mechanically moving the radar antenna. Electronic scanning with antenna at a fixed position (for example in phased array antennas) is usually implemented in ground-based radars or relatively large surveillance aircrafts.
Many aircrafts have a slotted planar array antenna that is located at the nose, belly, roof or tail of the aircraft and is protected by a radome. A slotted array antenna includes a slotted plane that includes a very large number of slots wherein each slot acts as a simple antenna element. The slotted plane is connected to a network of microwave waveguides. This antenna is moved by one or more servo-electrical motors. Typically, one servo-electrical motor performs horizontal scans while another servo-electrical motor performs elevation scans. The servo-motors are usually connected to a pedestal that in turn is connected to an aircraft airframe. Typically, a roll-and-pitch stabilized radar scan generator controls the scanning pattern of the radar antenna.
Many airborne radars are multi-mode radars. They can operate in several operational modes such as air-to-air modes, air-to-surface modes and sub-modes. Each mode may be characterized by a different scanning pattern. Thus, the field of view, angular scanning speed, and the number of bars can alter from mode to mode.
In addition, the characteristics of the transmitted radio frequency pulse sequences can vary from one mode to another. Some modes are characterized by low pulse repetition frequency (PRF), some are characterized by high pulse repetition frequency and others are characterized by medium pulse repetition frequency.
There is a need for systems and methods for generating virtual targets and especially for operating as non-intrusive systems.